# Basic probability

1. Mar 25, 2005

### JohanL

I have some simple questions about distributions in probability.
what are the distributions for X, Y and N(i) in these cases:

1. You have k urns and n balls. X is the number of balls in one urn.

2. You throw two dices untill you get a pair of sixes. X is the number of throws.

3. You have three batteris A,B and C. There durations (? how long they last. im from sweden) have exponential distributions with DIFFERENT parameters. If you first use A and B in a lamp what is the distribution for X, the time the
lamp works. Then you replace the nonworking battery with battery C. What is the distribution for Y, the time the lamp works after the exchange.

4. You throw one dice over and over again. Let N(i) be the throw when you get i for the first time. What is the distribution for N(i).

Thank you.

I have no problem to do the calculations when i know the distributions. The hard point for me is to figure out the distributions...my book isnt so good.

2. Mar 25, 2005

### matt grime

1. not enough information. you presumably uniformly distributed.

2 is geometric

3. scratch 3, that was wrong.

4. geometric again.

Last edited: Mar 25, 2005
3. Mar 25, 2005

### JohanL

Thx
But whats wrong with 3...its a problem from the book.

4. Mar 25, 2005

### matt grime

my answer was wrong, not the question.